Objective  Early human settlements are generally shaped by local environment, which is considered different in different places, especially in terms of regional identity. In China, traditional water towns in the Jiangnan region play a linking role between cities and villages, constantly inspiring and promoting regional urbanization. This research aims to explore the conventional paradigm of urban group arrangement in a particular locality. Specifically, the research reviews how traditional towns interact with local water environment at the regional scale, with a focus on the transformation of tacit wisdom into practicable principles for the current top-down requirement of high-quality construction and agglomerative development in tight coordination with the natural environment.

Methods  This research conducts an investigation on the geographical characteristics of water town groups based on historical materials. The research selects the northern Zhejiang plain as the research area for the following reasons. First, in the environmental aspect, the northern Zhejiang plain is a flat plate with high-density river networks, where there were more than two hundred historical water towns in ancient times. Second, in the socio-economic aspect, the plain area remains at a leading position nationwide in terms of social economy, having pioneering effects on other regions in China. All the major data involved in this research originate from an atlas of historical geological maps produced around 1916 to 1918 during the period of the Republic of China. This atlas shows in detail both natural elements and urban elements. By virtue of geographic information analysis tools, the research scans the two key variables of river course and town location as electronic records for further spatial analysis, with cities and towns being simplified as dots with location information and rivers as polygons with width information. Specifically, the research firstly investigates the geographic distribution of cities and towns through kernel density analysis. Then, it introduces an arteriality model to portray the multi-reach organization of river networks. After that, the research interprets and classifies the coupling relationship between town groups and river networks by morphological patterns in combination with the heterogeneous conditions of rivers and lands.

Results  Research findings can be summarized into the three features of "core-fringe", "settlement-river", and "town group-water network" respectively. For the spatial arrangement of cities and towns, the average distance between the center of town group and the adjacent county seated in the northern Zhejiang plain was roughly 18.3 km. It is indicated that the gathering of dozens of water towns gather in a small area can offset the negative impact caused by the large geographical distance and inconvenient traffic between counties and villages to some extent by exerting the scale agglomeration effect of town groups. For the organization of river networks, there existed an orderly organizational relationship between river networks under the arterial principle in the process of socioeconomic evolution. The Qiantang River, listed as a first-class river at the macro scale, flowed through Hangzhou City as the regional political and economic center of the northern Zhejiang plain; Yinxian City, Jiaxing City and other cities relied on Level-Ⅱ rivers within a certain drainage area such as the Yongjiang River and the Grand Canal; Level-Ⅲ rivers such as Jiashan Pond and Haiyan Pond served as the main intercity routes connecting Jiashan City and Haiyan City; Xincheng Town, Si'an Town and other towns generally had four river courses, with Xincheng Pond and Si'an Pond connecting the inter-town routes; one or more municipal rivers passing through a town not only formed the structural skeleton of the town but also served as the main channel between the town and the surrounding countryside. For the correlation between towns and rivers, there were a diversity of morphological patterns. In the coastal reclamation area where the water system was arranged in parallel, the towns were mostly located at the intersections between east-west rivers and north-south ones. Besides, in low-lying polder fields, rivers were arranged in the shape of network with towns distributed in the catchment of the main streams. Moreover, the mainstream of the radial river system was mostly artificially transformed, where the radiation center overlapped with the urban area, while towns were distributed in the suburbs. Finally, the water system of the piedmont plain extended naturally in the form of branches and veins as the terrain descends slowly, with river bifurcation nodes being occupied by towns.

Conclusion  In the process of regional environmental change and human-land relationship evolution over the past hundred years, Jiangnan water towns serve as not only the material continuation of the regional historical context but also the physical carrier of two-way selection and interaction between the built environment and the natural environment, which can give full play to the regional advantages of urban group development in environmental adaptation and dynamic adjustment. Cities and towns in northern Zhejiang fit well with the geographical environment and the original background of natural rivers therein and, in combination with local production and living needs, coordinate the hierarchy, scale and connection mode of the river network to adapt to social development, thus forming the organizational logic of progressive association between settlements at different scales and rivers at all levels.